Patients with asymmetric septal hypertrophy (ASH), concentric ventricular thickening, and normal septae were studied by ECG-gated scintigraphic angiocardiography to detect abnormalities characteristic of ASH. A lack of parallelism of septal surfaces, high ejection fraction, and decreased left ventricular compliance could be identified in most patients with, and no patients without, ASH. Furthermore, this technique probably provides the only reliable method for measuring ejection fraction in patients with ASH, and may therefore by useful in long-term studies of natural history in these patients.